1. Field of the Invention
This invention relates generally to a process and apparatus for solid state deformation of thermoplastic polymer billet shapes into angular shapes and to related products. One aspect of the invention relates to an extrusion die and process for extrusion of solid thermoplastic polymer billets into extruded angular shapes, preferably at high deformation rates.
Most polymers have a chain of carbon bonds along their backbone. Upon solidification of some polymers, a portion of the polymer chains in the material are folded to form crystals which are randomly oriented within the material. If even a small portion of the material behaves in this manner, the material is referred to as semi-crystalline. Such material may deform easily by bending, sliding and breaking of the crystals or a small fraction of the non-crystalline entangled molecular chains. If the chains are aligned or oriented, the mechanical strength is much improved. It is known that properties such as strength and stiffness are enhanced by aligning or orienting the polymer chains. One technique for orienting some polymers, such as polyethylene, is by plastic flow by solid state deformation at temperatures below the melting point.
Processes for the solid state deformation of polymers are well known. Among the processing techniques used to make profiles of polymers are ram and hydrostatic extrusion. In ram extrusion the billet of polymer is placed inside a usually cylindrical pressure chamber, so that the surface of the billet is in immediate contact with the walls of the chamber. One end of the chamber is fitted with a die, whose opening corresponds to the profile it is desired to produce. The other end of the pressure chamber is closed by an axially mobile ram, attached to a hydraulic system, so arranged that the ram pushes against the billet of the polymer and forces the polymer out from the chamber by flowing through the die.
In hydrostatic extrusion, the billet is much smaller than the pressure chamber, and the surface is separated by some distance from the chamber wall. The intervening space is filled with a hydraulic fluid. One end of the chamber is fitted with a pressure generated device, which may be a piston, or by an inlet through which hydraulic fluid is pumped into the chamber. The other end of the chamber is fitted with the die. One end of the billet is machined in such a way that the nose piece fits into the throat of the die, and makes a liquid tight seal. During extrusion, the pressure on the hydraulic fluid is increased. This pressure is transmitted in both the axial direction and the radial direction to the billet, so that it is pressurized equally in all directions. As a consequence, the surface of the billet is in contact with the oil, and some of this oil adheres to the surface of the billet as it passes through the die, providing a significant amount of lubrication.
During solid state deformation processes such as rolling, drawing and extrusion, the polymers lose the spherulitic or amorphous morphology generated during the cooling from the molten state, and become oriented usually in a longitudinal direction. The orientation of the polymer in a longitudinal direction increases the mechanical properties in the longitudinal direction of the polymer, e.g. its tensile strength and stiffness. These are sought after properties. One disadvantage of oriented polymers is that they are weak in the transverse direction and are subject to transverse cracking under stress.
In a process for forming a profile of substantially different cross-sectional shape than the polymer billet by extrusion through a die, the polymer is forced to undergo significant transverse flow at the same time as it is being oriented by longitudinal flow. In particular, the "transverse flow" as used herein means flow of polymer across a radial plane that contains the extrusion axis. If another form of deformation is used, transverse flow refers to flow of polymer across a radial plane that contains the deformation axis. Solid state deformation of a polymer profile shape usually takes place in one direction, and the deformation axis lies in that direction at the centroid of the cross-section or profile of the polymer shape being deformed. The shear stresses and strains generated during this transverse flow, particularly if the polymer is already oriented, result in the formation of cracks in the profile or to the generation of weak planes in the extruded profile that crack easily upon subsequent loading. The result is that when oriented polymer angular profiles of commercial interest, such as I-beams and channel sections, are extruded from cylindrical or rectangular polymer billets using conventional conical dies, serious flaws occur in the profiles, appearing as cracks or weak planes. These flaws become more severe in larger profiles (greater than about 1 cm.sup.2 cross-section) having angular or asymmetrical cross-sections.
The weak plane can be characterized as an internal surface or a plane within the polymer shape with very few tie molecules bridging the polymer lying on either side of the surface or plane. As a result, the interface at this location is very weak, allowing the cracks to form.
The present invention is preferably used when obtaining highly oriented polymers of large cross-section at high extrusion rates. For polyethylene, polypropylene and many other polymers of this invention, an oriented polymer is obtained beginning at a deformation ratio of greater than 5, or preferably greater than 8, and most preferably greater than 10.
Deformation ratio, as used herein, means the ratio of the cross-sectional area of the polymer shape before deformation to the cross-sectional area of the polymer shape after deformation and orientation. In the processes of this invention deformation ratios of 5 to 30 are preferable, with those in the range above 8 being more preferred, and those in the range above 10 being most preferred. These ratios of 8 to 10 and above are high deformation ratios in polymers such as polyethylene, which means that the polymer has become highly oriented as a result of deformation.
As used herein, "highly oriented" refers to the morphology of a polymer shape. Polymer shapes after cooling from the molten state have spherulitic morphology. After substantial deformation in the solid state, these shapes have a fibrillar morphology. Fibrillar morphology differs from "fibrillation," which refers to a kind of material failure. Fibrillar morphology in the case of polyethylene of the molecular weights used in the examples of this application begins to appear at a deformation ratio of about 5, at which point the polymer begins to become oriented. Fibrillar morphology is obtained in polyethylene of the molecular weights of the examples herein at deformation ratios of 8 to 10 and above. Polymer shapes with such a non-spherulitic, or a fibrillar, morphology are called "highly oriented". The deformation ratio required to achieve a highly oriented polymer will vary with the particular polymer involved and its molecular weight.
2. Description of the Related Art
Much research work has been carried out on extruding polymers, such as high density polyethylene, while in the solid state, utilizing dies with converging inner faces to convert round billets into round rods or threads. Examples of extrusion dies that utilize two stages to make a non-round shape are shown in Lo U.S. Pat. Nos. 4,789,514 and 4,877,393. Those patents disclose a die in which the surfaces converge in a first lateral direction and diverge in a second lateral direction, and in which the polymer is made to flow in the required directions through the action of protrusions on the working surfaces of the die.
A two stage die is described in Research Disclosure 18661 "Process and Die for Extruding Profiled Articles for Fibre Polymer Composites," published in Research Disclosure--October 1979. However, in this case the first die stage has a cross-sectional area less than about 25% of that of the finished profile, while the second die stage has a cross-sectional area approximately equal to that of the finished profile, but has a shape proportional to that of the first stage. Upon passing through the second stage, the material experiences an area expansion of at least 4:1.
It is the object of the present invention to provide a process and apparatus for solid state polymer deformation of angular profile from cylindrical or prismatical shapes without the creation of weak planes or cracks in the angular profile.